You can add an off-platform session by following these steps:
From the Quran, tap on the menu button in the header to open up the menu
Tap on the "Activity" tab of the scrollable tab list at the top of the screen
Tap on "Add Session"
Pro-tip: If you missed a day of recitation and lost your streak, you can restore it by manually adding an off-platform session for the day you missed (note that this is a Premium subscription feature only).